First-principles calculations of high pressure and temperature properties of Fe7C3

Abstract

Eckstrom–adcock iron carbide (Fe7C3) is considered to be the main constituent of the Earth’s inner core due to its low shear wave velocity. However, the crystal structure of Fe7C3 remains controversial and its thermoelastic properties are not well constrained at high temperature and pressure. Based on the first-principles simulation method, we calculate the relative phase stability, equation of state, and sound velocity of Fe7C3 under core condition. The results indicate that the orthorhombic phase of Fe7C3 is stable under the core condition. While Fe7C3 does reproduce the low shear wave velocity and high Poisson’s ratio of the inner core, its compressional wave velocity and density are 12% higher and 6% lower than those observed in seismic data, respectively. Therefore, we argue that carbon alone cannot completely explain the thermal properties of the inner core and the inclusion of other light elements may be required.